A lithium ion battery using a liquid electrolyte includes a negative electrode and a positive electrode divided by a separator, and thus may cause a short-circuit when the separator is damaged by deformation or external impact, which may lead to dangerous situations, such as over-heating or explosion. Therefore, development of a polymer electrolyte capable of ensuring safety may be an important subject in the field of lithium ion secondary batteries.
A lithium secondary battery using a polymer electrolyte provides increased safety to a lithium ion battery, prevents leakage of an electrolyte to provide improved reliability of a battery and facilitates manufacture of a thin battery, and thus is expected to be applied to a high-capacity secondary battery for electric vehicles as well as a compact secondary battery. Therefore, such a lithium secondary battery has been given many attentions as a next-generation battery.
However, a lithium secondary battery using a polymer electrolyte shows lower ion conductivity as compared to a liquid electrolyte and provides low output characteristics particularly at low temperature. In addition, since a solid electrolyte shows lower adhesion to the surface of an active material as compared to a liquid electrolyte, and thus provides increased interfacial resistance. The solid electrolyte is distributed while not being in contact with an electrode active material so that the output characteristics or capacity characteristics may be degraded as compared to the amount of a conductive material introduced to a battery. Further, an electrode using a solid polymer electrolyte has a lower content of active material in the electrode as compared to an electrode for a battery using a liquid electrolyte. Thus, it is required to increase the ratio of an active material to accomplish high energy density. Additionally, the portion of polymer electrolyte that is in direct contact with an electrode active material may be affected by the redox reaction of the electrode active material. In the case of a polymer electrolyte, it has lower redox stability and fluidity as compared to a liquid electrolyte and is fixed in its position, and thus the above-mentioned effects may be concentrated at a specific portion continuously to accelerate deterioration of the electrolyte. Therefore, there has been a limitation in developing a wide voltage battery by using a solid electrolyte according to the related art. FIG. 1 shows an electrode for a solid state battery including a solid polymer electrolyte according to the related art, and FIG. 2 is a schematic view illustrating partially enlarged FIG. 1. Referring to FIG. 1 and FIG. 2, the conductive material is contained in the solid electrolyte but a part of the solid electrolyte introduced to the battery cannot be in direct contact with the active material but is spaced apart from the active material since it has no fluidity. The remaining conductive material cannot participate directly in the electrochemical reaction upon the driving of the battery to cause degradation of output characteristics or capacity. For this reason, when using such a solid electrolyte, it is not possible to realize the capacity of the electrode sufficiently, unlike the electrode using a liquid electrolyte. As a result, the electrode using such a solid electrolyte provides a capacity lower than the designed or theoretical capacity.